The present invention relates to a ceramic multilayer wiring substrate and, more particularly, to the structure of through holes which are formed in such a wiring substrate.
A ceramic multilayer wiring substrate is extensively used as a wiring substrate for various kinds of electronic equipment. This kind of wiring substrate has a plurality of ceramic green sheets which are connected in a laminate configuration. Though holes are formed one in each green sheet in alignment with one another and are filled with conductive paste. A particular wiring pattern is formed on the surface of each green sheet and connected to the through hole of the sheet. The wiring patterns on the nearby green sheets are connected to each other by the through holes.
In the above structure, all the through holes interconnecting the wiring patterns provided on the individual green sheets have the same configuration, so that the conduction resistance is undesirably high. To eliminate this problem, the through holes may each be provided with a larger diameter or, alternatively, a plurality of through holes may be assigned to each of expected purposes. However, with the larger diameter scheme, it often occurs that the amount of buried conductive paste is short or the paste slips off. Such occurrences would prevent the wiring substrate from achieving a desired resistance despite the larger through holes. The multiple through hole scheme is not practicable unless a land provided on the surface of the wiring substrate for interconnecting the through holes is increased in size, obstructing dense arrangement of circuit elements on the surface of the substrate.